Implement type negation syntax

[PhoenixWhitefire]

Implements syntax as per negation-types RFC (luau-lang/rfcs#29)

• Adds flags LuauTypeNegationSyntax and LuauTypeNegationSupport
• Adds 1 parser test and 7 inference tests
• Adds InvalidNegation error

[PhoenixWhitefire]

Just need to convert it to use a negate internal type function and should be good

[PhoenixWhitefire]

Have updated it to use an internal negate type function as per RFC

[PhoenixWhitefire]

A PR has been made by Alex, so I will be closing this under the presumption that their implementation will be better.

#2242

[PhoenixWhitefire]

have nabbed CST from darkontas' PR